CIRCULATORY ORGANS. 275 
to the radix aorte (fig. 281). These two vessels parallel each other for 
a part of their course and are connected with each other by numerous 
capillary loops which run through the gill filaments. In passing 
through the gills the blood loses its carbon dioxide and takes up oxygen, 
and thus becomes converted from venous to arterial blood. In the am- 
niotes afferent and efferent branchial arteries are never differentiated, the 
aortic arches being continuous from ventral aorta to the radices aorte. 
The first of these arches (the mandibular arteries) never forms 
afferent and efferent portions since no gills are ever developed in their 
region. From each half of this arch an artery, the external carotid, 
da 
TT TC OTTO 
Fic. 281.—Scheme of branchial circulation in elasmobranchs. a, atrium; aa, afferent 
branchial arteries; av, abdominal vein; c, gill clefts; cc, common carotid; da, dorsal aorta; 
ea, efferent branchial arteries; hv, hepatic vein; ic, internal carotid; ec, external carotid 
artery; 7, jugular vein; /, liver; pc, postcardinal vein; sc, subclavian vein; sv, sinus venosus; 
tr, truncus arteriosus. 
extends forward to supply the lower and a part of a upper jaw, while an 
internal carotid artery forms an extension forward of each radix and 
supplies the brain and face. Later their relations are such that the 
carotids appear to arise from the first of the functional arches. 
The radices aorte of the two sides meet and fuse behind the last 
aortic arch, forming a single tube, the dorsal aorta, which runs in the 
middle line, dorsal to the alimentary tract, to the end of the body. The 
fusion may also extend forward from the last aortic arch, involving the 
whole of the radices. 
From the dorsal aorta segmental arteries extend laterally between 
the somites, these forming the upper halves of the transverse somatic 
vessels alluded to on page 268. To these the name of intercos- 
tal arteries, derived from human anatomy, is given. Ventral to them 
the aorta also gives off other arteries (nephridial arteries) to the excre- 
